1. Field of the Invention
The present invention relates to a radio system and, more particularly, to a space diversity system suitable for a fixed base station included in a digital mobile radio system of the type consuming little power.
2. Description of the Prior Art
In a digital radio system, interferential fading is apt to lower received power and distort waveforms to thereby aggravate the error probability. A space diversity system is an implementation available for coping with such fading. Generally, the term "diversity" is directed to a receiving system of the kind using two or more receivers which have little correlation with respect to interferential fading, i.e., which do not undergo deterioration in quality at the same time. The outputs of the two or more receivers are selectively used or mixed together to reduce the influence of interferential fading. The term "space diversity" refers to a system in which antennas are located spatially apart from each other to produce received input electric fields which have little correlation with respect to interferential fading. These concepts are described in "Digital Microwave Communication" supervised by Kuwabara, Kikaku Center, Japan, pp. 219-221, May 25, 1984.
Space diversity is generally classified into two types, i.e., selective space diversity which selects and outputs one of two received signals having higher quality than the other, and a mixing space diversity which mixes two received signals and outputs the resulting composite signal, as taught in the above document, pp. 221-225, for example. An actual system is designed by choosing the optimal one of the two different systems in consideration of the purpose of communication, reliability, etc.
However, with portable radio equipment intended for data transmission, it is difficult to use a plurality of antennas and receivers because such equipment has to be miniature and light weight. For this reason, a portable receiver is not practicable with the space diversity system and is, therefore, forced to accept the degradation in error probability ascribable to interferential fading.
To achieve higher data reliability in the above situation, data may be corrected, retransmitted or otherwise manipulated by detecting errors, as proposed in the past. These conventional approaches, however, are not effective when it comes to a critical degree of interferential fading.